This invention relates generally to air pumps, and more particularly to an air pump for generating BOTH positive and negative air pressure.
FIG. 1 shows a conventional air pump 1. The pump includes a pump body 3 comprising a cylinder 5, a piston 7 in the cylinder, and a hand-operated actuator 9 connected to a piston rod 11 for moving the piston 7 in the cylinder through a power stroke. A spring 13 is provided for moving the piston 7 through a return stroke following a power stroke. The pump 1 has a selector valve 15 rotatable between negative-pressure and positive-pressure positions. When the selector valve 15 is in its negative-pressure position, a negative pressure is generated at an outlet 17 of the pump as the actuator 9 is operated to move the piston 7 through a power stroke. When the selector valve 15 is in its positive-pressure position, positive pressure is not generated as the actuator 9 is operated to move the piston through a power stroke. Instead, positive pressure is generated during the return stroke of the piston 7. As a result, the amount of positive pressure generated is limited by the force exerted by the spring 13. This spring force is generally sufficient to generate a maximum positive pressure of about 15 psi. To create a larger positive pressure, it is necessary for the user of the pump to manually push the piston rod 11 to the left to provide a greater motive force to the piston 7.
Accordingly, there is a need for an improved pump design which allows the pump to generate both negative and positive air pressure during the power stroke of pump operation.